Recently, the recorder and recording medium, which can digitally record data, have been popular. Since video data and music data can be recorded to and played back from the recorder and recording medium without quality degradation, such data can be copied over and over again. Since such a digitally copied data keeps the very quality that the original data has, it will sell in the market. Because of this fact, however, the copyrighter of the video data or music data is afraid that his or her data will be copied many times and put on the market. In this circumstance, the recorder and recording medium are required to incorporate a feature of preventing copyrighted data from being illegally copied as above.
For the copyright protection, a method called “Serial Copy Management System (SCMS)” is applied for the mini-disc (MD) (traded mark). In this system, SCMS data is transmitted along with music data. It indicates that the music data is a one allowed to freely be copied (will, be referred to as “copy free”), a one allowed to be copied only once (will be referred to as “copy once allowed”) or a one prohibited from being copied (will be referred to as “copy prohibited”). When a mini-disc (will be referred to as “MD” hereinafter) recorder receives music data from a digital interface, it will detect the SCMS data having been transmitted along with the music data. If the SCMS data is “copy prohibited”, the MD recorder will not record the music data to the MD therein. If the SCMS data is “copy once allowed”, the MD recorder will change the SCMS data to “copy prohibited” and record the SCMS data along with the received music data to the MD. If the SCMS data is “copy free”, the MD recorder will record to the MD the SCMS data as it is along with the received music data.
Using the SCMS data as in the above, the MD system prevents copyrighted data from illegally being copied.
For prevention of copyrighted data from illegally being copied, another method is also available. It is called “content scramble system” and applied for the digital versatile disc (DVD) (trade mark). In this system, all copyrighted data in a disc are encrypted and only a licensed recorder is given an content key to decrypt the encrypted data for acquisition of meaningful data. For getting licensed, the recorder is designed to conform an operation prescription against illegal copying etc. Thus the DVD system prevents copyrighted data from illegally being copied.
In the method applied for the MD system, however, there may possibly be produced illegally a recorder which is not in conformity to the operation prescription that when the SCMS data is “copy once allowed”, it should be changed to “copy prohibited” and recorded along with received data.
The method adopted in the DVD system is effective on a read-only memory (ROM) medium, but not on a random-access memory (RAM) medium to which the user can record data. That is, even if the user cannot decrypt data in a RAM medium, he or she can illegally copy all data in the RAM medium in consideration to a new RAM medium which can be played in a licensed (legal) recorder.
Thus, the Applicant of the present invention proposed a technique for prevention of illegal copying. This technique will briefly be described in the following. Namely, data intended for identification of individual recording media (will be referred to as “medium identification data” hereinafter) is recorded in each recording medium to allow only a licensed recorder to access the medium identification data as disclosed in the Applicant's Japanese Patent Application No. 10-25310 (Japanese Published Unexamined Patent Application No. 11-224461, issued on Aug. 17, 1999). More specifically, data in a recording medium is encrypted using both a key based on a secret acquired through licensing and a medium identification data such that the data will be meaningless even if any unlicensed recorder can read it. Further, with this technique, when a license is granted to a recorder, the operation of the recorder is prescribed not to make any illegally copying. Thus, with the above technique, the unlicensed recorder cannot access the data and the medium identification data for each medium takes a unique value, so that even if the unlicensed recorder copies all accessible data to a new medium for example, a licensed recorder will not be able to correctly read the data from the new medium, whereby illegal copying can be prevented.
With the above conventional technique, however, to assure that a recording medium to which data has been recorded by a recorder can be read by another recorder, an content key for encryption of data in the recording medium is to be generated based on a common secret key (maser key) for the entire system. That is, if the master key is fraudulently stolen through analysis of a legal or licensed recorder, all data recorded by any recorder included in the system will possibly be decrypted and thus the system as a whole will be destroyed.